1. Field of the Invention
This invention relates generally to solar collector apparatus, and more specifically to a glass heat pipe evacuated tube solar collector and method of fabricating same.
2. Description of the Prior Art
Most solar collectors used today have flat absorbing surfaces with one or more flat glass covers to retard heat loss from the absorbing surfaces. Although a selective coating having high absorption and low emissivity characteristics on the absorber surface also aids in minimizing heat losses, conventional flat solar collectors still lose significant thermal energy to the atmosphere due to convection and conduction in the air space adjacent the absorber surface. It is well known that evacuating the region between the absorber and the glass cover further reduces heat loss due to conduction and convection; however, the structure of the glass forming the vacuum chamber must be strong enough to withstand the atmospheric pressure on the external surface of the glass cover. Therefore, one of the most suitable geometric shapes for evacuated collectors is tubular because of the inherent strength of a tubular structure to withstand such external pressure, U.S. Pat. No. 980,505 issued in 1911 to W. Emmet illustrates such a concentric evacuated tube collector.
While the evacuated envelope around tubular solar radiation absorbers greatly minimizes conduction and convection losses, the vacuum will not reduce energy loss due to infrared radiation emitted from the heated absorber. However, selective coatings on the absorber surface, which have good absorption characteristics for solar energy but which emit or re-radiate relatively little of the absorbed thermal energy, are well known and suitable for use in evacuated tube solar collector devices. In fact, conventional selective coatings in a vacuum do not have the degradation problems encountered with their use in flat plate solar collectors where they are exposed to the atmosphere and other working fluids.
Although evacuated tube solar collectors are not yet in general commercial use, significant developmental efforts have been made on them in the past several years. The principal developmental effort relating to evacuated tube solar collectors has been directed to methods and apparatus for removal of the thermal energy absorbed by the elongated glass tubes. One method of removing the thermal energy from the elongated inner glass tube of evacuated tube solar collectors is to circulate water or other working fluid into and out of the interior of the glass tube. The working fluid circulated through the glass tube absorbs the solar energy and carries it to a location where the energy can be stored or put to practical use. A number of patents illustrate this circulated working fluid absorber thermal energy removal technique, including: U.S. Pat. No. 980,505 issued to W. Emmet; U.S. Pat. No. 4,069,810 issued to H. Tabor; U.S. Pat. No. 4,018,215, issued to Y. Pei; U.S. Pat. No. 3,960,136, issued to K. Moan et al; U.S. Pat. No. 4,016,860, issued to K. Moan; U.S. Pat. No. 3,952,724, issued to Y. Pei; and U.S. Pat. No. 4,002,160, issued to G. Mather.
Another method of removing the thermal energy from the elongated inner glass tube of evacuated tube solar collectors is similar to those mentioned above, with the exception that the water or other working fluid is circulated through the elongated glass tube via pipes or circulation tubes positioned inside the glass tube so that the water or other working fluid does not actually come in contact with the glass tube. Examples of this method of thermal energy removal are shown in U.S. Pat. No. 4,080,954, issued to G. deWild, and German Pat. No. 2,612,171, issued to Philips.
Both of the design methods of solar energy removal from the glass tube absorber have the disadvantages of requiring pumps for circulating the working fluid through the elongated tube collectors thereby requiring additional energy consumption, excessive start-up time in the morning to heat the quantity of water necessary to function as a working fluid thereby reducing the effective working time of the collector panel, the working fluid in the collector is subject to freezing when the sun is not shining, and the working fluid in the tube is also capable of conducting thermal energy away from the object to be heated to the evacuated tube and radiating it to the exterior environment at night or during cold or cloudy weather thereby contributing to heat loss problems.
A third, relatively recent technique for removal of thermal energy absorbed by the elongated inner glass tube in evacuated tube solar collectors is to use heat pipes to transfer the absorbed solar energy to a working fluid medium that functions as a heat sink for storing the collected thermal energy or for transferring the energy to a location where it can be put to practical use. The evaporator portion of the heat pipe absorbs the solar energy, which causes a volatile thermal transfer fluid in the heat pipe (not the working fluid medium) to vaporize. The vapor pressure drives the vapor toward the cooler condenser section of the heat pipe, which is placed in contact with the working fluid medium or heat sink. The condenser portion of the heat pipe is where the thermal energy absorbed from the sun in the evaporator portion is conducted from the vapor of the thermal transfer fluid inside the heat pipe to the working fluid or heat sink outside the heat pipe. The lower temperature of the thermal transfer fluid vapor due to conduction of the heat from the vapor to the working fluid results in condensation of the thermal transfer fluid in the heat pipe. The condensed thermal transfer fluid then flows from the condenser portion back to the evaporator portion of the heat pipe where solar energy is absorbed to continue the cycle. An evacuated envelope is positioned around the evaporator portion of the heat pipe and a selective coating is placed on the exterior surface of the heat pipe to minimize convection, conduction, and radiation losses as described above. U.S. Pat. No. 4,059,093, issued to G. Knowles et al, is an example of a heat pipe evacuated tube solar collector.
The advantages of heat pipe collectors over the evacuated tube collectors utilizing other thermal energy removal techniques such as described above include, inter alia, the elimination of the risk of freezing liquid in the collectors, unidirectional heat flow from the absorber surface (the thermal diode effect), reduced pumping energy usage, lighter systems, and more rapid start-up or initial heat-up in the morning. However, the heat pipes utilized for solar collectors in the past have been metal heat pipes, such as that shown in the U.S. Pat. No. 4,059,093, issued to G. Knowles et al. In spite of the advantages mentioned above for heat pipe evacuated tube solar collectors, there are also a number of disadvantages associated with metal heat pipes in such applications that have not heretofore been solved. Some of the disadvantages include the requirement of maintaining a glass to metal vacuum seal to provide the transparent evacuated envelope around the evaporator portion of the heat pipe, the degradation problems associated with long term incompatibility of the metal with common working fluids, and expensive and only marginally effective wicks for the interior of the evaporator portion of the heat pipe. Effective vacuum sealing in a metal heat pipe is difficult and expensive to accomplish. The U.S. Pat. No. 4,082,575, issued to G. Eastman addresses some of the problems encountered in production of liquid compatible metals for heat pipes. Conventional mesh or grooved wicks in metal heat pipes leave much to be desired in efficient capillary drive of the thermal transfer fluids throughout the inside surface of the evaporator portions of the heat pipes due to only minimal surface contact between the wick and the surface of the heat pipe and nonoptimum porosity of commercially feasible wicks.